Method of forming nitridated tunnel oxide barriers for flash memory technology circuitry and STI and LOCOS isolation

ABSTRACT

An improved flash memory device, which has shallow trench isolation in the periphery region and LOCOS isolation in the core region is provided. A hard mask is used first to create the shallow trench isolation. The LOCOS isolation is then created. Subsequent etching is used to remove stringers. The flash memory is able to use shallow trench isolation to limit encroachment. The flash memory may also have a nitridated tunnel oxide layer. A hard mask is used to prevent nitride contamination of the gate oxide layer.

FIELD OF THE INVENTION

[0001] The present invention relates to nonvolatile memory devices. Even more particularly, the present invention relates to flash memory utilizing periphery and core stacks.

BACKGROUND OF THE INVENTION

[0002] Memory devices such as flash memory or electrically erasable programmable read only memory (EEPROM) are known. Memory devices such as flash memory comprise, core stacks, which hold the erasable programmable data, and periphery stacks which are used to program the core stacks. It is advantageous to manufacture periphery stacks and core stacks on the same chip, which is done in the prior art. However, sometimes it is desirable to use local oxidation of silicon (LOCOS) on part of the flash memory and shallow trench isolation (STI) on other parts of the flash memory.

[0003] In the case where shallow trench isolation is used for the periphery stacks, corner recesses form around the shallow trench isolation for the periphery stacks, which are detrimental to the periphery stacks.

[0004] In addition, core stacks and periphery stacks require different manufacturing steps. Some of these different processing steps for the core stacks are harmful to the periphery stacks and vice versa. One example of these problems is related to the use of a nitrogen implant or other nitridation methods to improve the functionality of the tunnel oxide of the core stacks. In the prior art, such a nitrogen implant tends to contaminate the gate oxide of the periphery stack diminishing the performance of the gate oxide of the periphery stacks.

[0005] It is desirable to manufacture periphery stacks and core stacks on a single chip and minimizing damage to the periphery stacks and core stacks due to different processes required to manufacture the different stacks.

[0006] In addition, it is desirable to have periphery stacks with gate oxides of different thicknesses.

BRIEF SUMMARY OF THE INVENTION

[0007] It is an object of the invention to provide a plurality of semiconductor devices on a single chip, where some of the devices are separated by shallow trench isolation and other devices are separated by local oxidation of silicon.

[0008] It is another object of the invention to provide flash memory, with a reduced contamination of the gate oxide.

[0009] It is another object of the invention to provide a flash memory device with improved stack isolation.

[0010] Accordingly, the foregoing object is accomplished by using successive hard masks to provide STI and LOCOS isolation on a single chip and then producing a flash memory device on a substrate by using a hard mask to protect the periphery, before forming a nitradated tunnel oxide.

[0011] Other features of the present invention are disclosed or apparent in the section entitled: “DETAILED DESCRIPTION OF THE INVENTION.”

BRIEF DESCRIPTION OF DRAWINGS

[0012] For a fuller understanding of the present invention, reference is made to the accompanying drawings wherein:

[0013]FIG. 1 is a cross sectional view of a semiconductor substrate used in a preferred embodiment of the invention;

[0014]FIG. 2 is a cross sectional view of the substrate shown in FIG. 1, with trenches;

[0015]FIG. 3 is a cross sectional view of the substrate shown in FIG. 2, with a trench oxide;

[0016]FIG. 4 is a cross sectional view of the substrate shown in FIG. 3, with corner recesses;

[0017]FIG. 5 is a cross sectional view of the substrate shown in FIG. 4, before LOCOS;

[0018]FIG. 6 is a cross sectional view of the substrate shown in FIG. 5, with LOCOS;

[0019]FIG. 7 is a cross sectional view of the substrate shown in FIG. 6, after the removal of the hard mask used for LOCOS;

[0020]FIG. 8 is a cross sectional view of the substrate shown in FIG. 7, with a hard mask layer;

[0021]FIG. 9 is a cross sectional view of the substrate shown in FIG. 8, with a tunnel oxide and first polysilicon layer;

[0022]FIG. 10 is a cross sectional view of the substrate shown in FIG. 9, with an interpoly dielectric layer;

[0023]FIG. 11 is a cross sectional view of the substrate shown in FIG. 10, with a first gate oxide layer;

[0024]FIG. 12 is a cross sectional view of the substrate shown in FIG. 11, where the first gate oxide layer is etched back;

[0025]FIG. 13 is a cross sectional view of the substrate shown in FIG. 12, after the photoresist mask has been removed;

[0026]FIG. 14 is a cross sectional view of the substrate shown in FIG. 13, with thin and thick oxide layers; and

[0027]FIG. 15 is a cross sectional view of the substrate shown in FIG. 14, with periphery stacks and core stacks.

[0028] Reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE OF THE INVENTION

[0029]FIG. 1 is a cross sectional view of a semiconductor substrate 10 used in a preferred embodiment of the invention. A pad oxide layer 12 is formed over a surface of the semiconductor substrate 10. A 1,000 to 2,000 Å first hard mask layer 14 is formed over the pad oxide layer 12. In the preferred embodiment of the invention, the first hard mask layer 14 is of the group consisting of silicon oxynitride (SiON), silicon nitride (Si₃N₄), and polysilicon. A photoresist mask 16 is formed over the first hard mask layer 14. Regions of the first hard mask layer 14 that are not covered by the photoresist mask 16 are etched away to form apertures 18 in the first hard mask layer 14, as shown in FIG. 1. In this embodiment the apertures 18 are only over the periphery region and interface region of the semiconductor substrate 10.

[0030] The photoresist mask is removed and the semiconductor substrate 10 is subjected to an etch, which creates shallow trenches 20 in the semiconductor substrate 10 below the apertures 18 in the first hard mask layer 14, as shown in FIG. 2. In the preferred embodiment, the depths of the trenches 20 into the substrate surface is approximately 0.15μ to 0.35μ. A trench oxide 22 is formed in the trenches, as shown in FIG. 3.

[0031] The semiconductor substrate 10 is then subjected to an etch to remove the first hard mask layer, as shown in FIG. 4. In the preferred embodiment the substrate 10 is then subjected to a cleaning step and the top of the trench oxide 22 has corner recesses 24 greater than about 50 Å deep. In the prior art, such corner recesses could be severely extended below the silicon surface.

[0032] A second hard mask 26, which in the preferred embodiment is about 1,000 to 2,000 Å thick is formed over the surface of the trench oxide 22 and pad oxide 12, as shown in FIG. 5. In the preferred embodiment of the invention, the second hard mask 26 is of the group consisting of silicon oxynitride (SiON), silicon nitride (Si₃N₄) and polysilicon. A photoresist mask is used to form apertures 28 in the second hard mask 26 over the core region and interface region of the substrate 10. The photoresist mask is then removed. The semiconductor substrate 10 is subjected to a clean step to remove greater than about 30 Å of oxide.

[0033] The semiconductor substrate 10 is then subjected to a low temperature oxidation at about 1050° C. to form LOCOS oxides 30, as shown in FIG. 6. The second hard mask 26 is then removed, and the remaining oxide is subjected to a clean step with an HF step to remove any remaining stringers in the oxide, as shown in FIG. 7. The semiconductor substrate 10 has STI isolation and LOCOS isolation on a single substrate and is ready for the manufacture of periphery and core stacks between the LOCOS oxides 30 and the trench oxide 22.

[0034] To begin the manufacture of the periphery and core stacks, a 100 to 500 Å third hard mask layer 42 is placed on the pad oxide 12 over both the periphery region and core region, as shown in FIG. 8. In the preferred embodiment of the invention, the third hard mask layer 42 is of the group consisting of silicon oxynitride (SiON), silicon nitride (Si₃N₄), and polysilicon. A photoresist layer is placed over the top surface of the third hard mask layer 42 and then etched back to form a photoresist mask 44 that does not cover the core section of the semiconductor substrate 10, as shown in FIG. 8. The trench oxide 22, pad oxide 12, and LOCOS oxide 30 are not drawn to scale so that more features may be shown.

[0035] The semiconductor substrate 10 is subjected to an etching process, which removes the third hard mask layer 42 and the pad oxide 12 over the core region, as shown in FIG. 9. The photoresist mask is then removed. A tunnel oxide layer 46 is formed over the core region. The tunnel oxide layer 46 may also be formed over the third hard mask layer 42. Various methods are known for forming the tunnel oxide layer 46, such as growing an oxide layer or depositing an oxide layer. In the preferred embodiment, the tunnel oxide layer 16 is nitridated (a nitride dopant is added to the tunnel oxide layer). There are various methods known for nitridating a tunnel oxide layer such as proving nitrogen dioxide (NO₂) during the oxidation process or implanting nitrogen into a tunnel oxide layer or in situ growth of NO_(x), where x=an integer. A first polysilicon layer 48 is formed over the tunnel oxide layer 46. A photoresist mask 49 is place over parts of the first polysilicon layer 48 over the core region.

[0036] The semiconductor substrate 10 is subjected to an etching process, which removes parts of the first polysilicon layer 48 and tunnel oxide layer 46, as shown in FIG. 10. The photoresist mask is removed. An interpoly dielectric layer 50 is formed over the substrate 10, third hard mask 42, and first polysilicon layer. In the preferred embodiment, the interpoly dielectric layer 50 is an oxide-nitride-oxide (ONO) layer. A photoresist mask 52 is formed over the interpoly dielectric layer 50 over the core region.

[0037] The semiconductor substrate 10 is then subjected to a two step etch that first removes the portion of the interpoly dielectric layer 50 over the periphery region and then removes the third hard mask 42 and the remaining pad oxide, as shown in FIG. 11. The photoresist mask 52 is then removed. The semiconductor substrate 10 is then subjected to a first thermal oxidation which forms a first gate oxide layer 54 over the semiconductor substrate 10 in the periphery region. In the preferred embodiment the first gate oxide layer 54 is about 100 Å. A photoresist mask 56 is formed over portions of the first gate oxide layer 54 in the periphery region and over the interpoly dielectric layer 50.

[0038] The parts of the first gate oxide layer 54 not covered by the photoresist mask 56 are etched away, as shown in FIG. 12. The photoresist layer 56 is then stripped away, as shown in FIG. 13 with the remaining first oxide layer 54 becoming the thick oxide regions 58. The semiconductor substrate 10 is then subjected to a second thermal oxidation, which forms thin oxide layers 60 in the uncovered regions of the substrate 10 and thick oxide layers 62 at the thick oxide regions 58, as shown in FIG. 14. In the preferred embodiment, the thin oxide layers 60 are 40 to 80 Å thick and the thick oxide layers 62 are 100 to 150 Å thick. A second polysilicon layer 64 is placed on the substrate 10, the thin oxide layers 60, the thick oxide layers 62 and the interpoly layer 50 and then etched back to form periphery stacks with thin gates 66, periphery stacks with thick gates 68, and core stacks 70, as shown in FIG. 15.

[0039] Conventional processes are then used to complete the flash memory structure. The inventive method allows the production of periphery gates with thin gates and thick gates to provide gates with different threshold voltages. In addition, core stacks with nitridated tunnel oxide layers are provided, without contaminating the gate oxide layers and allows the use of STI and LOCOS isolation on a single chip.

[0040] Information as herein shown and described in detail is fully capable of attaining the above-described object of the invention, it is understood that it is the presently preferred embodiment of the present invention and is thus representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.” 

We claim:
 1. A method for forming a plurality of semiconductor devices on a chip, comprising the steps of: forming a pad oxide layer over a surface of a substrate; forming a first hard mask layer over the pad oxide layer; forming a first photoresist mask over the first hard mask layer, wherein the first photoresist mask has a plurality of apertures; etching away parts of the first hard mask layer below the apertures in the first photoresist mask, to form apertures in the first hard mask layer; removing the first photoresist mask; etching trenches into the substrate below the apertures in the first hard mask layer; filling the trenches with trench oxide to provide shallow trench isolation; removing the first hard mask layer; forming a second hard mask layer over the pad oxide layer and trench oxide; forming a second photoresist mask over the second hard mask layer, wherein the second photoresist mask has a plurality of apertures; etching away parts of the second hard mask layer below the apertures in the second photoresist mask, to form apertures in the second hard mask layer; removing the second photoresist mask; oxidizing the substrate surface below the apertures in the second hard mask layer to form local oxidation of silicon isolation below the apertures in the second hard mask layer; and removing the second hard mask layer, wherein the first and second hard mask layers are from the group consisting of silicon oxynitride, silicon nitride, and polysilicon.
 2. The method, as recited in claim 1, further comprising the steps of: forming a third hard mask layer over the pad oxide layer; forming a third photoresist mask over the third hard mask layer, wherein the third photoresist mask has at least one aperture; etching away parts of the third hard mask layer and pad oxide layer under the at least one aperture in the third photoresist mask and leaving a remaining portion of the third hard mask layer; and forming a nitridated tunnel oxide layer over the substrate under the etched away part of the third hard mask layer.
 3. The method, as recited in claim 2, further comprising the steps of: forming a first polysilicon layer over the nitridated tunnel oxide layer; forming a fourth photoresist mask with a plurality of apertures over the first polysilicon layer; etching away parts of the tunnel oxide layer and first polysilicon layer under the apertures in the fourth photoresist mask; removing the fourth photoresist mask; and forming an interpoly dielectric layer over the third hard mask layer, substrate, and first polysilicon layer.
 4. The method, as recited in claim 2, further comprising the steps of: forming a fifth photoresist mask with an aperture over the remaining portion of the third hard mask layer; etching away the remaining portion of the third hard mask layer, wherein the third hard mask layer is from the group consisting of silicon oxynitride, silicon nitride, and polysilicon; removing the fifth photoresist mask; forming a first gate oxide layer over the surface of the substrate, which was under the remaining portion of the third hard mask layer; forming a sixth photoresist mask with apertures over portions of the first gate oxide layer; etching away portions of the first gate oxide layer under the apertures in the sixth photoresist mask; removing the sixth photoresist mask; and forming a second gate oxide layer over the surface of the substrate and the first gate oxide layer to form thick gate oxide regions and thin gate oxide regions.
 5. The method, as recited in claim 4, further comprising the steps of: forming a second polysilicon layer over the thick gate oxide regions, the thin gate oxide regions, and the interpoly dielectric layer; and etching away portions of the second polysilicon layer, the interpoly dielectric layer, thin gate oxide regions, and thick gate oxide regions to form periphery stacks with thin gate oxides, periphery stacks with thick gate oxides and core stacks with nitridated tunnel oxides.
 6. A semiconductor chip comprising a plurality of semiconductor devices, manufactured by the process comprising the steps of: forming a pad oxide layer over a surface of a substrate; forming a first hard mask layer over the pad oxide layer, wherein the first hard mask layer is from the group consisting of silicon oxynitride, silicon nitride, and polysilicon; forming a first photoresist mask over the first hard mask layer, wherein the first photoresist mask has a plurality of apertures; etching away parts of the first hard mask layer below the apertures in the first photoresist mask, to form apertures in the first hard mask layer; removing the first photoresist mask; etching trenches into the substrate below the apertures in the first hard mask layer; filling the trenches with trench oxide to provide shallow trench insolation; removing the first hard mask layer; forming a second hard mask layer over the pad oxide layer and trench oxide, wherein the second hard mask layer is from the group consisting of silicon oxynitride, silicon nitride, and polysilicon; forming a second photoresist mask over the second hard mask layer, wherein the second photoresist mask has a plurality of apertures; etching away parts of the second hard mask layer below the apertures in the second photoresist mask, to form apertures in the second hard mask layer; removing the second photoresist mask; oxidizing the substrate surface below the apertures in the second hard mask layer to form local oxidation of silicon isolation below the apertures in the second hard mask layer; and removing the second hard mask layer.
 7. The semiconductor chip, as recited in claim 6, further comprising the steps of: forming a third hard mask layer over the pad oxide layer; forming a third photoresist mask over the third hard mask layer, wherein the third photoresist mask has at least one aperture, wherein the third hard mask layer is from the group consisting of silicon oxynitride, silicon nitride, and polysilicon; etching away parts of the third hard mask layer and pad oxide layer under the at least one aperture in the third photoresist mask and leaving a remaining portion of the third hard mask layer; forming a nitridated tunnel oxide layer over the substrate under the etched away part of the third hard mask layer. forming a first polysilicon layer over the nitridated tunnel oxide layer; forming a fourth photoresist mask with a plurality of apertures over the first polysilicon layer; etching away parts of the tunnel oxide layer and first polysilicon layer under the apertures in the fourth photoresist mask; removing the fourth photoresist mask; forming an interpoly dielectric layer over the third hard mask layer, substrate, and first polysilicon layer; forming a fifth photoresist mask with an aperture over the remaining portion of the third hard mask layer; etching away the remaining portion of the third hard mask layer; removing the fifth photoresist mask; forming a first gate oxide layer over the surface of the substrate, which was under the remaining portion of the third hard mask layer; forming a sixth photoresist mask with apertures over portions of the first gate oxide layer; etching away portions of the first gate oxide layer under the apertures in the sixth photoresist mask; removing the sixth photoresist mask; and forming a second gate oxide layer over the surface of the substrate and the first gate oxide layer to form thick gate oxide regions and thin gate oxide regions.
 8. The semiconductor chip, as recited in claim 7, further comprising the steps of: forming a second polysilicon layer over the thick gate oxide regions, the thin gate oxide regions, and the interpoly dielectric layer; and etching away portions of the second polysilicon layer and the interpoly dielectric layer to form periphery stacks with thin gate oxides, periphery stacks with thick gate oxides and core stacks with nitridated tunnel oxides.
 9. A semiconductor chip, comprising: a substrate a plurality of semiconductor devices on a surface of the substrate; a plurality of trench oxides separating some of the plurality of semiconductor devices on the surface of the substrate; and a plurality of LOCOS oxides separating some of the plurality of semiconductor devices on the surface of the substrate.
 10. The flash memory device, as recited in claim 9, wherein some of the plurality of semiconductor devices are core memory devices and some of the plurality of semiconductor devices are periphery memory devices.
 11. The flash memory device, as recited in claim 10, wherein the core memory devices are separated by LOCOS oxides, and wherein the periphery memory devices are separated by trench oxides.
 12. The flash memory device, as recited in claim 11, wherein each core memory device of the core memory devices comprises: a nitridated tunnel oxide layer on the surface of the substrate; a first polysilicon layer on the nitridated tunnel oxide layer; an interpoly dielectric layer on the first polysilicon layer; and a second polysilicon layer on the interpoly dielectric layer.
 13. The flash memory device, as recited in claim 12, wherein each periphery memory device of the periphery memory devices, comprises: a gate oxide layer on the surface of the substrate, wherein the gate oxide layer is not nitridated and wherein part of the substrate surface under the gate oxide layer is not nitridated; a first polysilicon layer over the gate oxide layer. 